1. Field of the Invention
This invention relates to nanoparticles. More particularly, it relates to quantum dot microbeads having a surface shell.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
There has been substantial interest in the preparation and characterization of compound semiconductors in the form of particles with dimensions in the order of 2-50 nanometers (nm), often referred to as quantum dots (QDs), nanoparticles, or nanocrystals. Interest has arisen mainly due to the size-related electronic properties of these materials that can be exploited in many commercial applications such as optical and electronic devices, biological labeling, solar cells, catalysis, biological imaging, light-emitting diodes, general space lighting, and electroluminescent and photo-luminescent displays.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface-to-volume ratio: for smaller particles, the ratio of the number of surface atoms to the number of interior atoms is large. Thus, surface properties play an important role in the overall properties of the material. Also, the electronic properties of the material changes with size. For example, the band gap is larger for smaller particles because increased carrier confinement gives rise to discrete energy levels similar to those observed in atoms and molecules, rather than the continuous band of the corresponding bulk semiconductor material.
Among the most studied semiconductor quantum dot materials have been the chalcogenide II-VI materials, namely zinc sulfide (ZnS), zinc selenide (ZnSe), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe). Reproducible quantum dot production methods have been developed from “bottom-up” techniques, whereby particles are prepared atom-by-atom, i.e. from molecules to clusters to particles, using wet chemical procedures.
Coordination about the final inorganic surface atoms of nanoparticles may be incomplete, with highly reactive non-fully coordinated atomic “dangling bonds” on the surface of the particle. Such dangling bonds lead to particle agglomeration. The problem of agglomeration may be overcome by passivating (e.g., capping) the bare surface atoms with protective organic groups.
Single-core semiconductor nanoparticles are generally a single semiconductor material along with an outer organic passivating layer. Single-core nanoparticles tend to have relatively low quantum efficiencies because of non-radiative electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface.
One method to eliminate such defects and dangling bonds is growing a second inorganic material on the surface of the core particle to produce a “core-shell” nanoparticle. Generally, the second inorganic material has a wider bandgap than the core material and also has a small lattice mismatch to that of the core material. Core-shell nanoparticles separate carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. The small lattice mismatch between the core and shell materials also minimizes non-radiative recombination. One example of a core-shell nanoparticle is ZnS grown on the surface of CdSe cores.
Another approach to maximizing quantum efficiency is growing a core-multi shell structure where the electron-hole pair is completely confined to a single shell layer. In these structures, the core is of a wide bandgap material, surrounded by a thin shell of narrower bandgap material, and capped with a further wide bandgap layer, such as CdS/HgS/CdS. In such a structure, a few monolayers of mercury sulfide (HgS) are formed on the surface of the core CdS nanocrystal and then capped by additional CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the narrower bandgap HgS layer.
The outermost layer of organic material (i.e., the capping agent) helps to inhibit particle aggregation and also protects the nanoparticle from the surrounding chemical environment. In many cases, the capping agent is the solvent in which the nanoparticle preparation is undertaken, typically a Lewis base compound or a Lewis base compound diluted in an inert solvent such as a hydrocarbon. The capping agent can include a lone pair of electrons that are capable of donor-type coordination to the surface of the nanoparticle. Examples may include mono- or multi-dentate ligands of the types: phosphines (trioctylphosphine, triphenolphosphine, t-butylphosphine), phosphine oxides (trioctylphosphine oxide), alkyl phosphonic acids, alkyl-amine (hexadecylamine, octylamine), aryl-amines, pyridines, long chain fatty acids, and thiophenes. Alternatively, capping agent may include a functional group that bonds directly to the outermost inorganic layer. An example of such a capping agent is a thiol, wherein the —SH is capable of bonding to the QD.
The capping agent of a QD may also be a coordinated ligand that processes additional functional groups that can be used as chemical linkage to other inorganic, organic or biological material. Such functional groups may point away from the QD surface and be available to bond/react with other available molecules, such as primary, secondary amines, alcohols, carboxylic acids, azides, or hydroxyl groups. Moreover, the capping agent may include polymerizable functionalities, which may be polymerized to form a polymer around the QD particle.
The most widely studied quantum dots presently are based on cadmium-containing semiconductors such as CdS and/or CdSe. However, in many regions of the world there is now a restriction or ban on the use of heavy metals in many household goods which means that most cadmium-based quantum dots are unusable for consumer-goods applications.
It is thus commercially important to develop a range of heavy metal-free QDs that exhibit bright emissions in the visible and near infra-red region of the spectrum and that have similar optical properties to those of CdSe quantum dots. However, cadmium-free QD materials have proven to be more difficult to work with than their cadmium-containing counterparts. Specifically, cadmium-free materials are more sensitive to reactions with their environment, which cause a drop in quantum yield of the emission. It is thus desirable to develop systems that protect cadmium-free QDs from reacting with their environment.
One method of protecting QDs from their chemical environment has been to encapsulate the QDs within microbeads of a polymer material. Examples of microbead-encapsulated QDs are described in Applicant's U.S. Pat. No. 7,544,725, issued Jun. 9, 2009 and U.S. Pat. No. 7,674,844, issued Mar. 9, 2010, and in Applicant's Application Pub. Nos. U.S. 2011/0068321 and U.S. 2011/0084322, both published Mar. 24, 2011. The entire content of those patents and applications are incorporated herein by reference. While encapsulating QDs within polymer microbeads have shown to improve the stability and optical performance of the QDs, it has proven difficult to provide polymer systems that are both compatible with the QDs and that also efficiently protect the QDs from oxygen and other reactants. There is thus a need for further systems for protecting QDs from their environment.